Ion implantation has become the technology preferred by industry to dope semiconductors with impurities in the large scale manufacture of integrated circuits. A typical ion implanter comprises three sections or subsystems: (i) a terminal for outputting an ion beam, (ii) a beamline for directing and conditioning the beam output by the terminal, and (iii) an end station which contains a semiconductor wafer to be implanted by the conditioned ion beam. The terminal includes a source from which a beam of positively charged ions is extracted. The beamline components adjust the energy level and focus of the extracted positively charged ion beam on its way toward the wafer to be implanted.
A problem encountered in the use of such an ion implanter is that of wafer charging. As the positively charged ion beam continues to impact the wafer, the surface of the wafer may accumulate an undesirable excessive positive charge. Resulting electric fields at the wafer surface can damage microcircuitry on the wafer. The problem of accumulated surface charge becomes more pronounced as implanted circuit elements become smaller, because smaller circuit elements are more susceptible to damage caused by the resultant electric fields.
Another problem encountered in the use of such an ion implanter, especially in low energy applications, is a phenomenon referred to as beam "blow-up", which concerns the tendency for like (positively)-charged ions within the beam to mutually repel each other (also known as the space charge effect). Such mutual repulsion causes a beam of otherwise desired shape to diverge away from an intended beamline path. Beam blow-up is particularly problematic in high current, low energy applications because the high density of ions in the beam (high current) exaggerates the force of mutual repulsion of the ions, and the small velocities (low energy) of the ions allows more time for the repulsive force to act upon the ions before they reach the wafer.
A known solution to both wafer charging and the beam blow-up phenomenon is the use of an electron or plasma shower. Such showers may also be referred to as electron or plasma floods. Both electron and plasma showers generate low energy electrons and introduce these electrons into the beam. Plasma floods generate a plasma in an arc chamber and the ion beam potential extracts low energy plasma and electrons into the beam. Electron showers generate secondary (low energy) electrons which are used to enhance the beam to reduce space charge (beam blow-up) tendencies and wafer charging effects.
A typical electron shower includes a target chamber in which secondary electrons are generated and an extension tube connected downstream of the target chamber. As the ion beam passes through the target chamber, secondary electrons infiltrate and partially neutralize the beam. The partially neutralized beam passes through the extension tube toward the wafer to be implanted. The trapped low energy electrons thereby neutralize the net charge of the beam which in turn reduces the positive charge accumulation on wafer as the ion beam strikes the wafer surface. The neutralized beam is also less likely to experience detrimental beam blow-up characteristics. Such a system is shown in U.S. Pat. No. 4,804,837 to Farley, assigned to the assignee of the present invention and incorporated by reference as if fully set forth herein.
Secondary electrons are generated within the target chamber as follows. A filament within the target chamber is electrically heated so that it thermionically emits primary high energy electrons into the chamber. These primary electrons strike the inner surface of the target chamber which emits secondary electrons as a result of the impact. The target chamber is typically comprised of aluminum due to its high secondary electron yield. In addition, an inert gas such as argon or xenon is introduced into the target chamber and ionized, by means of collisions between inert gas molecules and the high energy electrons, to produce an ionized plasma. The presence of the ionized plasma enhances generation of secondary electrons by increasing the extraction rate of secondary electrons from the inner surface of the target chamber.
Because the high energy primary electrons produced by the filament possess too great an energy to be captured within the ion beam, they do not assist in reducing the tendency of beam blow-up as do the secondary electrons. In addition, primary electrons which reach the surface of the wafer can adversely negatively charge the wafer surface, possibly to the extent of causing damage to the wafer. It is therefore desirable that the primary electrons emitted by the filament become expended through collisions with either the inert gas molecules or the inner surface of the target chamber.
Additionally, in known electron shower target chambers constructed of aluminum, oxidation at the inner surface thereof creates an aluminum oxide coating. High energy electrons which impact the aluminum oxide cause a negative electrical potential to develop, which increases with the thickness of the oxide coating. Secondary electrons which are emitted from the coated surface as a result of the impacting high energy electrons assume energy levels consistent with the oxide potential. The energy level of the secondary electrons therefore drifts higher and higher as the oxide coating continues to thicken over time.
The effectiveness of a particular electron shower depends in part on the efficient generation of a sufficient supply of secondary electrons having consistently low and predictable energy levels. Accordingly, it is an object of the present invention to provide an electron or plasma shower which increases its secondary electron emission current while insuring that secondary electron energy levels are minimal and constant.
It is a further object of the present invention to provide an electron or plasma shower in which high energy primary electrons are efficiently utilized to create low energy secondary electrons, by directing the high energy electrons toward a desired target chamber surface and toward an inert gas stream introduced into the chamber, to maximally exhaust the supply of primary high energy electron and enhance the generation of low energy secondary electrons.
It is still a further object of the present invention to provide an electron or plasma shower having a target chamber constructed from a material which reduces oxidation on the surface thereof, to provide better control over the energy levels of secondary electrons emitted therefrom.
It is yet a further object of the present invention to provide an electron or plasma shower having a target chamber having a configuration which resists back-sputtered contamination and which increases both the volume and density of an ionized plasma contained therein.